Memory is one type of integrated circuitry, and is used in computer systems for storing data. Memory may be fabricated in one or more arrays of individual memory cells. Memory cells may be written to, or read from, using digit lines (which may also be referred to as bit lines, data lines, or sense lines) and access lines (which may also be referred to as word lines). The sense lines may conductively interconnect memory cells along columns of the array, and the access lines may conductively interconnect memory cells along rows of the array. Each memory cell may be uniquely addressed through the combination of a sense line and an access line.
Memory cells may be volatile, semi-volatile, or nonvolatile. Nonvolatile memory cells can store data for extended periods of time in the absence of power. Nonvolatile memory is conventionally specified to be memory having a retention time of at least about 10 years. Volatile memory dissipates, and is therefore refreshed/rewritten to maintain data storage. Volatile memory may have a retention time of milliseconds or less. Regardless, memory cells are configured to retain or store memory in at least two different selectable states. In a binary system, the states are considered as either a “0” or a “1”. In other systems, at least some individual memory cells may be configured to store more than two levels or states of information.
The smallest and simplest memory cell will likely be comprised of two electrodes having a programmable material, and possibly a select device (such as a diode or ovonic threshold switch), received between them. Suitable programmable materials have two or more selectable memory states to enable storing of information by an individual memory cell. The reading of the cell determines which of the states the programmable material is in, and the writing of information to the cell places the programmable material in a predetermined state. Some programmable materials retain a memory state in the absence of refresh, and thus may be incorporated into nonvolatile memory cells.
Arrays of memory cells may comprise a plurality of access lines at one elevation and a plurality of sense lines at another elevation, with at least programmable material there-between. Individual memory cells are written to or read from by application of suitable voltage and/or current to the respective crossing access line and sense line. Some arrays of memory cells include two or more elevationally stacked tiers or decks of memory cells, with each tier comprising a plurality of access lines at one elevation, a plurality of sense lines at another elevation, and at least programmable material there-between. The tiers or decks are typically fabricated separately and sequentially, one atop another.